(1) FIELD OF THE INVENTION
The present invention relates to a process of removing sulfur dioxide from waste gases and more particularly to a process of removing sulfur dioxide from waste gases by absorption, to a composition useful in removing such sulfur dioxide-containing waste gases, and to a process of recovering sulfur from said composition after its use.
(2) DESCRIPTION OF THE PRIOR ART
A number of processes for solving the problem of removing sulfur dioxide from waste gases which may contain, among other impurities, hydrogen sulfide in amounts up to 1% as well as oxygen, have been suggested heretofore. However, many of such processes cannot be used economically because they yield waste products which do not find any further use. An efficient process for recovering sulfur dioxide from waste gases must allow recirculation of the absorbent substantially without impairing its activity. Suitable absorbents are buffer systems. As is known, buffer systems represent equilibria which can be displaced towards absorption or, respectively, towards desorption when used for solving the present problem. In addition thereto and in accordance with their buffering action, their pH-value is only slightly changed when adding thereto or, respectively, removing therefrom an acid, i.e., in the present case, sulfur dioxide SO.sub.2.H.sub.2 O. This property can be of considerable importance with respect to the suppression of side-reactions.
Thus a number of recently suggested absorption processes of removing sulfur dioxide from waste gases are based on the use of such buffer systems. For instance, German Published Application No. 2,208,102 discloses a process which is governed by the equilibrium EQU Na.sub.2 SO.sub.3 +SO.sub.2 +H.sub.2 O.revreaction.2NaHSO.sub.3 (1)
and is based by way of the equation EQU HSO.sub.3.sup.- +H.sub.2 O.revreaction.SO.sub.3.sup.2+ +H.sub.3 O.sup.+( 2)
on the buffer system EQU H.sub.3 O.sup.+ =K . [HSO.sub.3 '/SO.sub.3 "] (3)
The process disclosed in U.S. Pat. No. 2,031,802 is also based upon a buffer system because citric acid used therein is only partly neutralized by means of an alkali metal compound.
In German Published Application No. 2,250,959, there is described the use of the buffer system EQU HPO.sub.4 "/H.sub.2 PO.sub.4 '
for the absorption of sulfur dioxide.
Regeneration of the absorption solution can be effected in various ways. When using the citrate process, the equilibrium is displaced by considerably increasing the temperature, i.e. by evaporation. In the two other processes described hereinabove, the sulfur dioxide is reacted to yield sulfur by introducing hydrogen sulfide into the absorption solution while increasing the temperature, if required.
All these processes have the disadvantage that they are not reversible to a satisfactory degree. A considerable amount of the absorbed sulfur dioxide is not desorbed nor converted into sulfur but it reacts with the formation of by-products such as the sulfate which cannot be recovered at all or which can be worked up to useful products only with difficulty and incompletely such as the thiosulfate or, respectively, the corresponding thionates.
The reason why such by-products are formed are manifold and can be indicated only briefly hereinafter. Formation of sulfate is mainly due to three facts. On the one hand the action of oxygen on sulfur dioxide causes oxidation, the speed of oxydation increasing when the pH-value increases from a pH of 2.0 to a pH of 6.0, the maximum speed of oxidation being observed at a pH of about 6.0.
Another cause of sulfate information is to be seen in the sulfite being disproportioned to sulfate and sulfur. Such disproportioning is favorably effected by a high sulfur dioxide concentration and high temperature. High sulfur dioxide concentration and high temperature, however, are conditions which are prevalent, for instance, at the beginning of the regeneration phase in the above mentioned buffer absorption solution.
A third possibility of sulfate formation consists in the reaction of pyrosulfite (S.sub.2 O.sub.5 ") with the information of dithionate and sulfate. This reaction takes place especially at low pH values, for instance, at a pH of 2.0 to 3.0, and at high sulfur dioxide concentrations, i.e. under conditions as they prevail at the end of the absorption phase if no special countermeasures are taken.
The above mentioned known processes, however, have further disadvantages. Thus, there are formed, especially in the presence of finely divided sulfur, polythionate (at a low pH-value) or thiosulfate (near the neutral point).
Another disadvantage of the known processes consists in the difference between the absorption temperature and the boiling point of the solution being relatively small. However, in order to achieve satisfactory regeration, a sufficient increase in temperature is essential so as to set free sufficient amounts of sulfur dioxide from the HSO.sub.3 ' ion which sulfur dioxide subsequently reacts further or, respectively, is desorbed.